1. Field of the Invention
The present invention relates to transmitting data over existing cable television plants using cable modems. More specifically, it relates to reducing noise leakage from a cable modem when the modem is not transmitting data and providing matched termination to the cable plant whether or not the cable modem is active.
2. Discussion of Related Art
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable systems now include fiber optics as part of its signal transmission component. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cablecommuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable head end, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the head end was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers. Branching off of these trunk lines were distribution or feeder cables (40% of the system's cable footage) to specific neighborhoods, and drop cables (45% of the system's cable footage) to homes receiving cable television. Amplifiers were provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the head end of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 60 to 700 MHz. Broadcast signals were sent downstream; that is, from the head end of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscribers. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the head end, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the head end of the cable system and distribution nodes (discussed in greater detail with respect to FIG. 1 below). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. In many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting systems are known as hybrid fiber-coaxial (HFC) systems. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and of other frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers, whereas a typical coaxial trunk line is about 10 kilometers, as mentioned above. Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary purpose was transmitting a variety of entertainment television signals to subscribers. Thus, they needed to be one-way transmission paths from a central location, known as the head end, to each subscriber's home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas between 500 to 2,000 subscribers. Although networks using exclusively fiber optics would be optimal, presently cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using only fiber optic cables.
FIG. 1 is a block diagram of a two-way hybrid fiber-coaxial (HFC) cable system utilizing a cable modem for data transmission. It shows a head end 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Head end 102 contains a cable modem termination system (CMTS) 104 that is needed when transmitting and receiving data using cable modems. Head end 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each head end can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the head end and each distribution node. In addition, because cable modems were not used, the head end of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110 to two-way amplifiers or duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service up to 500 subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is presently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 33,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data may be transferred at speeds up to 10 million bps. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
TABLE 1Time to Transmit a Single 500 Kbytes ImageTelephone Modem (28.8 KBPS)6-8minutesISDN Line (64 KBPS)1-1.5minutesCable Modem (10 Mbps)1second
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional PC data modem, provides this high speed connectivity and is, therefore, instrumental in transforming the cable system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable industry has been upgrading its coaxial cable systems to HFC systems that utilize fiber optics to connect head ends to fiber nodes and, in some instances, to also use them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and faster than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers decrease a cable system's channel capacity, degrade the signal quality, and are susceptible to high maintenance costs. Thus, distribution systems that use fiber optics need fewer amplifiers to maintain better signal quality.
Digital data on the upstream and downstream channels is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert digital data to a modulated RF signal and convert the RF signal back to digital form. The conversion is done at two points: at the subscriber's home by a cable modem and by a CMTS located at the head end. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal. Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cablecommuting (a group of workers working from home or remote sites whose numbers will grow as the cable modem infrastructure becomes increasingly prevalent). Not surprisingly, with the growing interest in receiving data over cable network systems, there has been an increased focus on performance, reliability, and improved maintenance of such systems. In sum, cable companies are in the midst of a transition from their traditional core business of entertainment video programming to a position as a full service provider of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
A problem common to upstream data transmission on cable systems, i.e. transmissions from the cable modem in the home to the head end, is ingress noise at the head end which lowers the signal-to-noise ratio, also referred to as carrier-to-noise ratio. Ingress noise can result from numerous internal and external sources. Sources of noise internal to the cable system can include cable television network equipment, subscriber terminals (televisions, VCRs, cable modems, etc.), intermodular signals resulting from corroded cable termini, and core connections. Significant sources of noise external to the cable system include home appliances, welding machines, automobile ignition systems, and radio broadcast. All of these ingress noise sources enter the cable system through defects in the coaxial cable line, which acts essentially as a long antenna. Ultimately, when cable systems are entirely optical fiber, ingress noise will be a far less significant problem. However, until that time, ingress noise is and will continue to be a problem with upstream transmissions.
In addition to ingress noise (external sources of noise to a cable plant), another source of noise on the upstream channel is transient noise leakage from a cable modem itself. A cable modem contains, among several other components, an upstream transmitter coupled with a variable amplifier. FIG. 2 is a block diagram of a cable modem and subscriber end of a cable plant. A cable modem 202 transmits data on upstream channel 204. An upstream transmitter 206 is connected to an amplifier 208 by a control line 210 which allows transmitter 206 to enable or disable amplifier 208 (the two components are also connected via a data line 212 for the transmission of data packets). When cable modem 202 is turned on, upstream transmitter 206 enables amplifier 208 and when it is turned off, the transmitter disables the amplifier.
A problem occurs because control line 210 often transmits control messages from transmitter 206 to amplifier 208 at a speed that does not give the amplifier sufficient time to enable or turn on. This can cause interference or collision of data packets during transmission. For example, upstream transmitter 206 informs amplifier 208 via the control line 210 that the amplifier should expect data in a certain amount of time, such as in two microseconds. Amplifier 208 then has two microseconds to enable itself before it begins receiving data. With most variable amplifiers, the “warning” time is typically not sufficient to prepare for receiving data packets. Consequently, data packet loss can occur. The amplifier then transmits the data through a diplex filter 214 which acts as an entry/exit point for data for cable modem 202. Also shown is a downstream receiver 216 for receiving data on the downstream channel through diplex filter 214.
Because variable amplifier 208 in a cable modem often cannot transmit data packets at a sufficient speed, it is kept enabled via the control line, even when the cable modem is not transmitting data. By keeping the variable amplifier within the cable modem enabled in such a manner, a cable modem can create noise as a result of leakage from upstream transmitter 206 and variable amplifier 208. Ideally, when a cable modem is turned off and the variable amplifier is disabled, there is complete isolation and no noise emanates from the modem. Complete isolation implies there is no leakage of power from the amplifier and, thus, no noise created by the modem on the upstream channel. However, it is difficult to obtain complete isolation for a cable modem.
Because control line 210 keeps the variable amplifier persistently enabled as instructed by upstream transmitter 206, there is typically a small amount of leakage at all times. While power leakage from one or two cable modems is not noticeable, a significant noise problem occurs when many modems in a service area having hundreds of cable modems, often referred to as a system of modems, have leakage. Leakage from the modems accumulates at the head end and raises the noise floor of the upstream spectrum. Generally, leakage from all the cable modems accumulates arithmetically to increase the noise floor of the entire upstream channel. This reduces the overall signal-to-noise ratio of the upstream spectrum. This, in turn, significantly limits the number of cable modems able to use an upstream channel in accordance with the DOCSIS standard which requires that the signal-to-noise ratio be no less than 44 dB.
Presently, a solution for reducing noise on an upstream spectrum is to partition a service area so that the new service areas contain fewer cable modems. By partitioning a service area, an upstream channel used by one of the new service areas will have less accumulated noise from the cable modem themselves. However, this solution is impractical and inefficient as service areas grow and the number of cable modem subscribers increases.
Therefore, it would be desirable to be able to reduce or eliminate leakage from a cable modem thereby lowering the cumulative noise floor of the upstream channel. It would also be desirable to achieve this increased isolation of a cable modem without having to keep the modem or any components within the modem enabled at all times, even when not in use.